Just use the % (modulo) operator. It returns the remainder of a division. For example:
28 % 24 = 4
In the case of the example I gave you, The last line sets the DaysTillBDay equal to:
Date.ToTotalHours(abs(Date.Difference(Date.Parse(Date.ToDateString(Date.Now)), Date.Parse(right(NextBirthDay.Text, 11))))) / 24
For the remaining hours, you would just set it equal to:
Date.ToTotalHours(abs(Date.Difference(Date.Parse(Date.ToDateString(Date.Now)), Date.Parse(right(NextBirthDay.Text, 11))))) % 24
Even if you didn't know about the % operator though, you could just do this:
totalHours - (DaysTillBDay * 24)